An example of a conventional method of defect inspection by eddy currents using electromagnetic induction is disclosed in JP-B 3753499 (Patent Document 1). Patent Document 1 describes arrangements to detect at least one of an output voltage difference and a phase difference between detection signals induced in two detection coils oriented perpendicularly with respect to an excitation coil, thereby increasing the detected output voltage and enabling the damage conditions such as cracks in the surfaces of steel frames covered with fire resistant coatings inside buildings to be inspected without stripping away the coating material. However, with such an arrangement, a skin effect causes the magnetic field to decrease exponentially in the depth direction of the sample (steel frame), so damage can only be detected at the surface of the sample (steel frame) covered by the coating material, and it is not possible to inspect for damage inside or on the back surface of the sample. This problem is not limited to Patent Document 1, and while many types of eddy current defect inspection apparatus, eddy current defect inspection methods and defect inspection probes (sensors) using electromagnetic induction have been proposed, it is a serious problem that is common to all of them.
Additionally, as another example of a conventional inspection device, JP-B 3266128 (Patent Document 2) discloses an invention for performing defect inspection by detecting leaked magnetic flux. In Patent Document 2, a plurality of magnetic sensors are arranged along the direction of magnetization of a magnetizer, leaked magnetic flux corresponding to the same positions on the sample is detected, and the measurement results for a plurality of magnetic sensors are computed, thereby reducing noise and detecting internal defects inside the sample. However, with such an arrangement, the end surface of a magnetizer is oriented not roughly perpendicular to the sample surface, but along the sample. However, in that case, only a portion of the magnetic field of the magnetizer is applied to the sample, moreover in a diagonal direction with respect to the sample, so the magnetic field of the magnetizer is not effectively applied to the sample. More importantly, the detection sensor is positioned on the back side of the sample, so the detection sensor detects the magnetic field (leaked magnetic flux) that has passed through the sample, but the magnetic field is exponentially reduced in the depth direction of the sample due to the skin effect, as a result of which the leaked magnetic flux is extremely weak. Additionally, a plurality of magnetic sensors must be provided and results measured by changing the magnetization conditions must be computed, making inspection considerably complicated. When performing inspection by detecting leaked magnetic flux in this manner, the leaked magnetic flux becomes extremely weak due to the skin effect for the reasons described above, thus inevitably limiting the inspection. In the examples of Patent Document 2, the objects being inspected are 1 mm-thick steel sheets, and the lift-off (distance between sample and detection sensor) is only 1 mm. Thus, in devices that perform inspection by detecting leaked magnetic flux, the skin effect limits what can be inspected to thin materials, making them entirely inapplicable to defect detection inside or on the back side of thick materials.
Additionally, as another example of a conventional inspection device, a proposal to perform defect inspection using the transmitted magnetic flux transmitted through the interior of a sample is disclosed in JP-A 2010-48552 (Patent Document 3). In Patent Document 3, defects inside a sample are detected by measuring the magnetic potential difference between two points of the transmitted magnetic flux transmitted through the interior of a sample (although Patent Document 3 uses the expression “magnetic potential”, there is interlinkage from the coil through which current applied to the magnetic circuit flows, as shown in FIG. 1 and FIG. 3 which is a simplified version thereof, so it cannot be treated like electrical potential, and the concept of magnetic potential as used in Patent Document 3 is physically meaningless). The detection principles are shown in the schematic diagram of FIG. 3 in Patent Document 3, and are explained as follows. In other words, the excitation core, the sample and the pickup core can be considered to constitute a pathway for magnetic flux. The magnetic flux applied from the excitation core to the sample flows through this pathway, and if there is a defect inside the sample, a large magnetic resistance will occur in the sample, so the output of the magnetic potential difference measuring means (detection coil) provided in the pickup core will change. As a result, so it is explained, defects inside the sample can be detected. However, the magnetic field (magnetic flux) applied from the excitation core to the sample will not flow uniformly inside the sample due to the skin effect, being greatest at the surface of application to the sample and exponentially decreasing toward the inside of the sample. As long as a normal alternating magnetic field is used, a transmitted magnetic flux that flows uniformly through the inside of the sample such as shown in FIG. 3 of Patent Document 3 will not exist. The applied magnetic field has its maximum intensity at the surface of the sample. While the frequency of the applied magnetic flux can be lowered by controlling the attenuation of the magnetic flux inside the sample to some degree, it is extremely difficult to pass a uniform transmitted magnetic flux through a sample. Even if it were somehow possible to apply and transmit an ultralow frequency much lower than 1 Hz, the detected output voltage of the coil is proportional to the square of the applied frequency, so the detection efficiency would be drastically reduced. For example, if the detected output at 1 kHz is 1 V, then the output at 0.1 Hz will be 0.01 μV, which would be drowned out by noise and make the detected signal very difficult to handle. Furthermore, the ultralow frequencies require the electronic circuits in the device to be compatible with ultralow frequencies, and make the data capture time longer, making fast measurements impossible, which is impractical. Thus, inspection using transmitted magnetic flux has the problem that the skin effect prevents the transmitted magnetic flux from flowing uniformly inside the samples, making it inapplicable to defect inspection inside or on the back surface of samples.
Additionally, as another example of a conventional inspection device, in JP-B 3896489 (Patent Document 4), foreign articles such as metal conductors contained in the object of measurement are detected by applying an alternating magnetic field to the object of measurement and studying the magnetic response signal of the magnetic sensor. The detection signals of the magnetic sensor are composed of the magnetic field that the applied alternating magnetic field creates at the position of the magnetic sensor and the magnetic field due to eddy currents generated by a metal conductor contained in the object of measurement (having a 90 degree phase lag with respect to the applied alternating magnetic field). When detecting foreign articles such as metal conductors, the magnetic field created by the applied alternating magnetic field at the position of the magnetic sensor becomes noise. By removing this alternating magnetic field with a cancellation coil, the detection sensitivity for the magnetic field due to eddy currents generated in metal conductors can be raised, enabling inspection for the presence or absence of foreign articles such as metal conductors contained in the object of measurement. Accordingly, in Patent Document 4, the magnetic field created by the applied alternating magnetic field at the position of the magnetic sensor is removed by a cancellation coil, thereby raising the detection sensitivity of the magnetic field due to eddy currents generated by metal conductors contained in the object of measurement, so the purpose is to inspect for the presence or absence of foreign articles such as metal conductors, and not to inspect the inside of metal conductors contained in the object of measurement.